P-type nitride semiconductor and method of manufacturing the same

ABSTRACT

A method for manufacturing p-type nitride semiconductor comprising a semiconductor layer forming process where a low resistivity p-type nitride semiconductor layer is formed on a substrate by introducing the sources of p-type dopant, nitrogen and Group III sources on a substrate held at a temperature of 600° C. or higher and a cooling process for cooling the substrate which is bearing the p-type nitride semiconductor layer. The manufacturing method features in that the hole carrier concentration of the p-type nitride semiconductor layer decreases during the cooling process. A superior quality p-type nitride semiconductor is made available, without needing any annealing treatment after growth, by properly specifying the concentration of atmosphere gas and the cooling time.

FIELD OF THE INVENTION

The present invention relates to a p-type nitride semiconductor, amongthe gallium nitride-based(Group III-V) semiconductor for use in lightemitting devices emitting blue light or other light of short wavelength;more specifically, a p-type nitride semiconductor that does not requireannealing treatment after growth. A method for manufacturing thesemiconductor is also included in the present invention.

BACKGROUND OF THE INVENTION

Gallium nitride-based(Group III-V) semiconductor, which has a relativelylarge bandgap, is one of the prospective materials suitable for theshort wavelength light emitting devices used in optical informationprocessing units handling the increasing amount of information contents.In such light emitting devices as a diode device or a laser device, a PNjunction is the essential structure, where the carriers are recombinedat the vicinity of the junction and the light is emitted. As is wellknown, it is not easy to provide a low resistivity nitride semiconductorbecause, in the p-type nitride semiconductor doped with magnesium, Mg,or other acceptor, the activation rate the acceptor is significantlylower relative to doner.

The p-type nitride semiconductor exhibits a high resistivity value evenwhen it is restored to room temperature after growth. In order to obtaina low resistivity, it has been a normal practice to apply a postannealing or other thermal treatment on a p-type nitride semiconductorto dissociate the hydrogen of a complex formed of magnesium and hydrogenfrom magnesium. Research activities are being made to provide a lowresistivity p-type nitride semiconductor without applying the postannealing. If it turns out to be successful, it will bring about anadvantage also for an improved productivity in such devices.

In the U.S. patent Publication 5,932,896 (Japanese Patent Laid-openNo.135575/1998), for example, a method for manufacturing a p-typenitride semiconductor without applying the post annealing is disclosed.

The method disclosed in the above Publication uses a Metal-OrganicChemical Vapor Deposition (MOCVD) process to grow a p-type nitridesemiconductor on a sapphire substrate; introducing an organic magnesiumcompound containing such Group III source as trimethylgallium (TMG),such nitrogen source as ammonia (NH₃) and p-type dopant on the substrateof 1100° C. using a nitrogen carrier gas containing hydrogen gas at a0.8%-20% concentration in the capacity percent. In this way, formationof a magnesium-hydrogen complex is blocked and a p-type nitridesemiconductor that exhibits a low resistivity during growth stage isprovided. It also discloses a cooling process, where the temperature isdecreased to 350° C. in an atmosphere of nitrogen gas containing ammoniaby approximately 32% in the capacity percent, and then supply of ammoniais suspended, and the temperature is lowered to room temperature.

The above describe conventional method for manufacturing a p-typenitride semiconductor eliminating the post annealing, however, hasfollowing problems. Namely, as the inventor of the above described U.S.Pat. No. 5,932,896 and other writer taught in a thesis (Applied PhysicsLetters, vol. 72, (1998), p.1748), the activation rate of magnesiumsignificantly deteriorates if the hydrogen concentration increasedmerely from 2.4% to 3.7% during crystal growth process, which means thata p-type nitride semiconductor is obtainable only when it is grown in avery low hydrogen concentration. What is more, if a p-type nitridesemiconductor is grown in a low hydrogen concentration the surfacemigration turns out to be insufficient, and certain specific atoms arenot disposed at respective optimum points on the surface, making itdifficult to obtain a good crystal.

SUMMARY OF THE INVENTION

The present invention addresses the above-described problems, and aimsto offer a superior quality p-type nitride semiconductor without needingthe post annealing treatment.

The method for manufacturing p-type nitride semiconductor comprises aprocess of forming a low resistivity p-type nitride semiconductor in thegrowing process, and a cooling process under which the cooling time orthe atmosphere is controlled so that the property of low resistivity ismaintained within a practical range usable as a p-type semiconductor.

Described concretely, in a method for manufacturing p-type nitridesemiconductor of the present invention, p-type dopant source, nitrogensource and Group III source are introduced on a substrate kept at atemperature 600° C. or higher to form a low resistance p-type nitridesemiconductor layer on the substrate, and the substrate bearing thep-type nitride semiconductor layer is cooled. During the coolingprocess, the hole carrier concentration in the p-type nitridesemiconductor layer decreases.

By so doing, a low resistance p-type nitride semiconductor layer isformed on a substrate in an atmosphere containing specific amount ofhydrogen that suppresses the p-type dopant from being inactivated, andthe hole carrier concentration of the p-type nitride semiconductor layerdecreases to a level at which the low resistivity property can bemaintained. In this way, a p-type nitride semiconductor having asuperior crystal quality is made available without needing any postannealing treatment.

In the present method for manufacturing p-type nitride semiconductor, itis preferred that in the cooling process the hole carrier concentrationof the p-type nitride semiconductor layer decreases to approximately0%-95%.

For example, supposing the hole carrier concentration immediately aftergrowth to be approximately 2.0×10¹⁷ cm⁻³, a concentration ofapproximately 1.0×10¹⁶ cm⁻³ can be maintained even if the hole carrierconcentration decreased by 95%. Thus, a p-type nitride semiconductorthat practically functions well is provided.

In the present method for manufacturing p-type nitride semiconductor, itis preferred that it contains in the cooling process a procedure forlowering the substrate temperature from the growth temperature toapproximately 600° C. within 30 min.

This makes it sure for the hole carrier concentration of a p-typenitride semiconductor layer to maintain the low resistivity property tobe sufficient for a practical functioning.

In the present method for manufacturing p-type nitride semiconductor, itis preferred that the atmosphere for semiconductor layer formationcontains hydrogen for approximately 5%-70% in capacity percent.

When an organometallic material is used as the source of Group IIIelement or p-type dopant, hydrogen is normally contained in theatmosphere for raising the decomposition efficiency and expediting thesurface migration. However, if the hydrogen concentration is beyond 70%,the inactivating effect caused by the hydrogen on acceptor becomessignificant. The inactivation of p-type dopant can be surely suppressedwhen the hydrogen concentration is controlled to be 5%-70% in accordancewith the present invention.

In the present method for manufacturing p-type nitride semiconductor, itis preferred that the atmosphere introduced during the cooling processuntil the substrate temperature reaches approximately 600° C. from thegrowth temperature contains hydrogen for approximately 0%-50% incapacity percent.

By so doing, the inactivation in a p-type nitride semiconductor due tohydrogen can be suppressed, and the low resistivity property ofrelatively high hole carrier concentration in a p-type nitridesemiconductor layer is well maintained.

In the present method for manufacturing p-type nitride semiconductor, itis preferred that the atmosphere introduced during the cooling processuntil the substrate temperature reaches approximately 600° C. from thegrowth temperature contains ammonia, NH₃.

By so doing, dissociation of nitrogen from the surface of the grownp-type nitride semiconductor can be suppressed, as a result,deterioration of the surface can be prevented.

In order to implement the earlier-described objective, the presentmanufacturing method forms a low resistivity p-type nitridesemiconductor in the p-type nitride semiconductor layer formationprocess, and then provides a certain specific contrivance on the coolingprocess covering a certain specific substrate temperature range, namely,a substrate temperature range approximately 950° C.-700° C., where theinactivation of p-type dopant emerges in the p-type nitridesemiconductor layer.

Described concretely, in the present method for manufacturing p-typenitride semiconductor, the p-type nitride semiconductor layer is cooledduring the above-described specific substrate temperature range under acertain specific condition where the inactivation of p-type dopant ishard to occur, which specific condition being built up of a combinationof the hydrogen concentration in atmosphere and the cooling time.

In the present method for manufacturing p-type nitride semiconductor,the p-type nitride layer is cooled during the above-described specificsubstrate temperature range under a certain specific condition where theinactivation of p-type dopant is hard to occur, which specific conditionbeing built up of a combination of the hydrogen concentration inatmosphere and the cooling rate.

This makes it possible for a p-type nitride semiconductor to maintainthe low resistivity property within a certain range, where it canfunction as a practical p-type semiconductor.

Further, a p-type nitride semiconductor of the present invention relatesto a p-type nitride semiconductor formed on a substrate at a growthtemperature of 600° C. or higher, where the hole carrier concentrationimmediately after the cooling process is approximately 5%-100% of thatat the growth temperature.

In a case where the hole carrier concentration is approximately 2.0×10¹⁷cm⁻³ immediately after the growth, a concentration of approximately1.0×10¹⁶cm⁻³ can be provided even if the hole carrier concentrationdecreased to 5% of that immediately after growth. Thus, it provides ap-type nitride semiconductor that functions well in practical usage.

Other p-type nitride semiconductor of the present invention relates to ap-type nitride semiconductor formed on a substrate one after another atthe growth temperature 600° C. or higher, the p-type nitridesemiconductor being exposed at the uppermost surface, the hydrogenconcentration at the vicinity of upper surface being the same or withinapproximately 10 times that inside the p-type nitride semiconductor.

In the conventional p-type nitride semiconductor manufactured throughthe processes accompanied by the post annealing treatment, hydrogenconcentration at the vicinity of exposed upper surface is greater bymore than 10 times that inside the p-type nitride semiconductor.However, in a p-type nitride semiconductor of the present invention, thehydrogen concentration at the vicinity of the upper surface of p-typenitride semiconductor remains at the same level, or within approximately10 times, as that inside of the p-type nitride semiconductor. Therefore,in accordance with the present invention, a p-type nitride semiconductorhaving an improved activation rate with the p-type dopant is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Cross sectional view showing the structure of a p-type nitridesemiconductor in embodiment 1 of the present invention.

FIG. 2 Graph showing dependence of the hole carrier concentration on thecooling time during cooling process, in a method for manufacturingp-type nitride semiconductor in embodiment 1 of the present invention.

FIG. 3 Cross sectional view showing a structure of p-type nitridesemiconductor in embodiment 2 of the present invention.

FIG. 4 Graph showing the distribution in depth direction of the hydrogenconcentration, with nitride semiconductor light emitting devices inembodiment 2, including a first modification and a comparative sample,of the present invention.

FIG. 5 Graph showing dependence of the hole carrier concentration on thetemperature of holding the substrate, during the cooling process in amethod for manufacturing p-type nitride semiconductor in an embodimentof the present invention.

FIG. 6 Graph showing dependence of the hole carrier concentration on thetime for cooling the substrate from approximately 950° C. toapproximately 700° C., during the cooling process in a method formanufacturing p-type nitride semiconductor in an embodiment of thepresent invention.

FIG. 7 Graph showing relationship between the hydrogen concentration inthe atmosphere and the time for cooling the substrate from approximately950° C. to approximately 700° C., during the cooling process in a methodfor manufacturing p-type nitride semiconductor in an embodiment of thepresent invention.

FIG. 8 Graph showing relationship between the hydrogen concentration inthe atmosphere and the substrate cooling rate at approximately 800° C.,during the cooling process in a method for manufacturing p-type nitridesemiconductor in an embodiment of the present invention.

FIG. 9 Cross sectional view showing the structure of a p-type nitridesemiconductor in other embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are described with reference to thedrawings.

Embodiment 1

A first exemplary embodiment of the present invention is described withreference to the drawings.

FIG. 1 is a cross sectional view showing the structure of a p-typenitride semiconductor in accordance with a first exemplary embodiment ofthe present invention. On a substrate 11 made of sapphire, a bufferlayer 12 formed of gallium nitride (GaN) for easing the lattice mismatchbetween a certain semiconductor to be grown on the substrate 11 and thesapphire, and a p-type nitride semiconductor layer 13 formed of GaN arestacked in the order.

Method for manufacturing the above p-type nitride semiconductor layer isdescribed in the following. In the first place, the substrate 11 havinga mirror-finished main surface is placed in a reaction chamber (notshown) and held by a substrate holder, and then temperature of thesubstrate 11 is raised to approximately 1000° C., and hydrogen gas isintroduced on the substrate 11 while it is heated for approximately 10min. Stains of organic substance and humidity sticking on the mainsurface are thus removed.

The substrate temperature is lowered to approximately 550° C., and thennitrogen gas is introduced as carrier gas at a flow rate ofapproximately 16 Liter/min., and ammonia, NH₃, gas as the source ofnitrogen at a flow rate of approximately 4 L/min., trimethylgallium(TMG) as the source of Group III at a flow rate of approximately 40μmol/min. on the substrate 11. The buffer layer 12 of GaN is thus grownon the main surface of substrate 11 for a thickness of 25 nm.

The TMG supply to reaction chamber is once suspended and the substratetemperature is raised to approximately 1050° C.; and a 2 μm thick p-typenitride semiconductor layer 13 of Mg doped GaN is grown on the bufferlayer 12, by introducing nitrogen gas at a flow rate of approximately 13L/min., hydrogen gas at a flow rate of approximately 3 L/min. as carriergas, and ammonia, NH₃, gas at a flow rate of approximately 4 L/min., TMGat a flow rate of approximately 80 μmol/min. andbiscyclopentadienymagnesium (Cp₂Mg) containing magnesium as p-typedopant at a flow rate of approximately 0.2 μmol/min. on the substrate 11for approximately 60 min. The above flow rate of hydrogen gas includesthe hydrogen gas needed for vaporizing the TMG and the Cp₂Mg.

The TMG and Cp₂Mg supplies to the reaction chamber are suspended, andthen the substrate 11 is cooled from the growth temperature down to roomtemperature while nitrogen gas at a flow rate of approximately 13L/min., hydrogen gas at a flow rate of approximately 3 L/min. andammonia, NH₃, gas at a flow rate of approximately 4 L/min. are beingintroduced as ambient gas on the substrate 11. After the cooling isfinished, the substrate 11 bearing p-type nitride semiconductor layer 13is taken out of the reaction chamber.

Now in the following, specific features contained in the cooling processof the present invention are described, which cooling process to beapplied on the substrate 11 bearing p-type nitride semiconductor layer13.

FIG. 2 is a graph showing dependence on the cooling time of the holecarrier concentration during the cooling process in a method formanufacturing p-type nitride semiconductor in the first exemplaryembodiment of the present invention. The hole carrier concentration wasmeasured with the p-type nitride semiconductor layers 13 having fivedifferent cooling time spans from 5 min. to 40 min. for lowering thesubstrate temperature from 1050° C., or growth temperature, to 600° C.Measurement of the hole carrier concentration was conducted by measuringthe hall effect with a 5 mm square sample chip prepared by separatingthe substrates 11 provided specifically for the measurement in fivedifferent cases.

As seen from FIG. 2, all of the five sample chips exhibit the p-typeconduction, and the hole carrier concentration decreases with thosehaving a longer time taken for cooling from the growth temperature to600° C. By extrapolating the straight line of FIG. 2 to the y piece forthe zero cooling time, it may be stated that the p-type nitridesemiconductor layer 13 of the present embodiment exhibits a p-typeconduction approximately 2×10¹⁷ cm⁻³ with the hole carrier concentrationimmediately after it is grown.

FIG. 2 also teaches us that: the hole carrier concentration with the 5min. cooling time is 1.2×10¹⁷ cm⁻³, that with the 20 min. cooling timeis 3.0×10¹⁶ cm⁻³, which corresponds to approximately 7% of theconcentration before the cooling, and that with the 30 min. cooling timeis 1.0×10¹⁶ cm⁻³, which corresponds to approximately 5% of theconcentration before the cooling. The case with the 30 min. cooling timeis almost at the bottom limit that can be used a p-type layer in adevice. In the case with the 40 min. cooling time, the concentration is2.2×10¹⁵ cm⁻³, indicating that the carrier concentration is insufficientto be used in a device.

First Modification of Embodiment 1

Method for manufacturing p-type nitride semiconductor layer inaccordance with a first modification example of embodiment 1 isdescribed below.

First buffer layer 12 and p-type nitride semiconductor layer 13 havingthe hole carrier concentration of approximately 2×10¹⁷ cm⁻³ are formedin the order on the substrate 11, as shown in FIG. 1, through the samemethod as in embodiment 1.

Dependence, during the cooling process in the first modificationexample, of the hole carrier concentration on concentration of hydrogengas contained in the ambient gas is described below. The hole carrierconcentration was measured with those which have undergone fourdifferent atmospheres having different hydrogen gas concentration, 0%,30%, 50% and 70%. Concentration of ammonia, NH₃, gas in each of theatmosphere was approximately 20%, and remainder of nitrogen gas. Thesubstrate was cooled from approximately 1050° C., or the growthtemperature, to approximately 600° C. in approximately 5 min.

Results of the measurement are:

1) 0% hydrogen concentration approximately 2×10¹⁷ cm⁻³, identical tothat immediately after the growth

2) 30% hydrogen concentration 4.2×10¹⁶ cm⁻³

3) 50% hydrogen concentration approximately 1×10¹⁶ cm⁻³, identical toapproximately 5% of that immediately after the growth

4) 70% hydrogen concentration approximately 2.5×10¹⁵ cm⁻³, identical toapproximately 1% of that immediately after the growth

As seen in the above, when cooled in an ambient of 70% hydrogenconcentration, the p-type layer becomes inadequate for use in a device,even if it is cooled to 600° C. in a 5 min. time span.

Second Modification of Embodiment 1

Method for manufacturing p-type nitride semiconductor layer inaccordance with a second modification example of embodiment 1 isdescribed below.

First buffer layer 12 and p-type nitride semiconductor layer 13 havingthe hole carrier concentration of approximately 2×10¹⁷ cm⁻³ are formedin the order on the substrate 11, as shown in FIG. 1, through the samemethod as in embodiment 1.

Dependence, during the cooling process in the second modificationexample, of the hole carrier concentration on concentration of ammonia,NH₃, gas contained in the atmosphere is described. Concentration ofhydrogen gas in the atmosphere was approximately 15%, and remainder ofnitrogen gas. The substrate was cooled from approximately 1050° C., orthe growth temperature, to approximately 600° C. in approximately 5 min.

As a result of the measurement, it has been confirmed that there ishardly any difference in the rate of decrease during the cooling processin the hole carrier concentration even when concentration of theammonia, NH₃, gas was varied; it stays at a level corresponding that of20% ammonia, NH₃, gas concentration. When the ammonia, NH₃, gasconcentration is within a range 0%-0.5%, the crystal propertydeteriorates due to dessociation of nitrogen from the surface of p-typenitride semiconductor layer 13.

Third Modification of Embodiment 1

Method for manufacturing p-type nitride semiconductor layer inaccordance with a third modification example of embodiment 1 isdescribed below. In the present modification example, dependence of thehole carrier concentration, during formation of the p-type nitridesemiconductor layer 13 shown in FIG. 1, on concentration of hydrogen gasin the carrier gas is described. In embodiment 1, concentration ofhydrogen gas in the carrier gas was approximately 15%; while, in thepresent modification example, concentration of the hydrogen gas incarrier gas was varied in five steps at a 5% interval from 0% to 20%, insix steps at a 10% interval from 20% to 80%, so, eleven steps in all. Inthe case of 0% hydrogen gas concentration, TMG and Cp₂Mg were vaporizedrespectively using nitrogen gas.

Result of the measurements on each of the p-type nitride semiconductorlayers 13 grown in an atmosphere where the hydrogen gas concentrationfalls within a range 5%-70% indicated that the hole carrierconcentration immediately after the growth was 1×10¹⁶ cm⁻³ or more,meaning that they have the p-type property; the value was obtained likewhat was shown in FIG. 2 by extrapolating the zero cooling time. Moreconcretely, the hole carrier concentration was approximately 5×10¹⁶ cm⁻³or more with the hydrogen gas concentration approximately 5%-50%,approximately 1×10¹⁷ cm⁻³or more with the hydrogen gas concentrationapproximately 10%-20%. When it is grown in a 15% hydrogen gasconcentration, among other cases, the hole carrier concentrationimmediately after the growth exhibits its peak value, as high as 2×10¹⁷cm⁻³.

With respect to the full line-width at half the maximum of rocking curvein X-ray diffraction, which represents a criteria for evaluating thecrystal property, it goes smaller along with the increasingconcentration of hydrogen gas; the full line width at half the maximumvalue is less than 300 sec. when concentration of the hydrogen gas is10% or higher. However, when it is grown in an atmosphere of zerohydrogen gas concentration, the full line-width at half the maximum ofrocking curve in X-ray diffraction becomes 500 sec. to a substantiallydeteriorated crystal property. Furthermore, the high resistivity makesit impossible to measure the hole carrier concentration.

In the same token, when it is grown in an atmospere of approximately 80%hydrogen gas concentration, the high resistivity makes it impossible tomeasure the hole carrier concentration. The reason for that issupposedly in an increased quantity of hydrogen atom taken during thegrowth process into the GaN crystal, which lowers the rate of activationwith the magnesium.

Although ammonia, NH₃, has been used in the present embodiment asnitrogen source, other organic nitride materials such as hydrazine,N₂H₄,ethylazide,C₂H₅NH₂, may also be used for the purpose.

Embodiment 2

A second exemplary embodiment of the present invention is described withreference to the drawings.

FIG. 3 is a cross sectional view showing the structure of a nitridesemiconductor light emitting device in accordance with a secondexemplary embodiment of the present invention. The epitaxial layer of anitride semiconductor light emitting device in the second embodimentcomprises a buffer layer 22 of non-doped GaN disposed on a sapphiresubstrate 21, an n-type contact layer 23 of Si-doped GaN, a lightemitting layer 24 of non-doped InGaN, a first clad layer 25 of non-dopedGaN, a p-type second clad layer 26 of Mg-doped AlGaN, and a p-typecontact layer 27 of Mg-doped GaN, stacked in the order. On the p-typecontact layer 27 is a light-transmitting positive-side electrode 28,which is formed of Ni and Au layers stacked thereon in the order. Anegative-side electrode 29 of Al is formed on the exposed region of then-type contact layer 23. Thus the present light-emitting device is alight-emitting diode having a PN junction of the n-type contact layer 23and the second clad layer 26, with the non-doped light-emitting layer 24and the first clad layer 25 interposed in between.

Method for manufacturing the light emitting diode of the above structureis described in the following.

In the first place, a substrate 21 having a mirror-finished main surfaceis placed in a reaction chamber (not shown) and held by a substrateholder, and then temperature of the substrate 21 is raised toapproximately 1000° C., and hydrogen gas is introduced on the substrate21 while it is heated for approximately 10 min. Stains of organicsubstance and humidity sticking on the main surface are thus removed,and a clean surface is provided.

The substrate temperature is lowered to approximately 550° C., and thennitrogen gas is introduced as carrier gas at a flow rate ofapproximately 16 L/min., ammonia, NH₃, gas as the source of nitrogen ata flow rate of approximately 4 L/min., and TMG as the source of GroupIII at a flow rate of approximately 40 μmol/min. on the substrate 21;the buffer layer 22 is thus formed with GaN on the main surface ofsubstrate 21 for a thickness of 25 nm. The flow rate of the hydrogen gasin the carrier gas includes the hydrogen gas needed for vaporizing theTMG or the Cp₂Mg.

The TMG supply to reaction chamber is once suspended and the substratetemperature is raised to approximately 1050° C., and a 2 μm thick n-typecontact layer 23 of Si doped GaN is grown on the buffer layer 22, byintroducing nitrogen gas at a flow rate approximately 13 L/min.,hydrogen gas at a flow rate of approximately 3 L/min. as carrier gas,and ammonia, NH₃, gas at a flow rate of approximately 4 L/min., TMG at aflow rate of approximately 80 μmol/min. and 10 ppm monosilane, SiH₄,containing silicon, an n-type dopant, at a flow rate of approximately 10cc/min. on the substrate 21 for approximately 60 min.

The TMG and SiH₄, gas supplies are suspended, and then the substratetemperature is lowered to approximately 750° C. At this growthtemperature, an SQW light emitting layer 24 of 3 nm thick InGaN is grownon the n-type contact layer 23, by introducing nitrogen gas as thecarrier gas at a flow rate of approximately 14 L/min., ammonia, NH₃, gasat a flow rate of approximately 6 L/min., TMG at a flow rateapproximately 4 μmol/min. and trimethylindium (TMI), other source ofGroup III, at a flow rate of approximately 5 μmollmin. on the substrate21. The In composition in the light emitting layer 24 of the presentcase is approximately 0.2.

Supply of the TMI is suspended, while the nitrogen carrier gas, theammonia, NH₃, gas as the nitrogen source and the TMG as the source ofGroup III are kept on flowing at the same flow rate respectively on thesubstrate 21. In this way, a first clad layer 25 of GaN is grown on thelight emitting layer 24 for a thickness of 10 nm in the course of thetemperature increase in the substrate to approximately 1050° C.

After the substrate temperature reaching at approximately 1050° C.,nitrogen gas at a flow rate of approximately 13 L/min., hydrogen gas ata flow rate of approximately 3 L/min. are introduced as carrier gas, andammonia, NH₃, gas at a flow rate of approximately 4 L/min., TMG at aflow rate of approximately 40 μmol/min., trimethylaluminum (TMA) asother source of Group III at a flow rate of approximately 6 μmol/min.and Cp₂ Mg at a flow rate of approximately 0.1 μmol/min. on thesubstrate 21 for growing a second clad layer 26 of Mg doped p-type AlGaNon the first clad layer 25 for a thickness of 0.2 μm.

After suspending the TMA supply, and keeping the substrate temperatureat approximately 1050° C., nitrogen gas at a flow rate of approximately13 L/min., hydrogen gas at a flow rate of approximately 3 L/min. areintroduced as the carrier gas, and ammonia, NH₃, gas at a flow rate ofapproximately 4 L/min., TMG at a flow rate of approximately 80 μmol/min.and Cp₂ Mg at a flow rate of approximately 0.2 μmol/min. on thesubstrate 21 for growing a p-type contact layer of Mg doped p-type GaNon the second clad layer 26 for a thickness of 0.3 μm.

After the p-type contact layer 27 is formed, substrate temperature islowered from the growth temperature to room temperature whileintroducing nitrogen gas at a flow rate of approximately 13 L/min.,hydrogen gas at a flow rate of approximately 3 L/min. and ammonia, NH₃,gas at a flow rate of approximately 4 L/min. in the reaction chamber asambient gas. The substrate 21 bearing an epitaxial layer formed of aplurality of nitride semiconductor layers is taken out of the reactionchamber. In the ambient gas, the hydrogen gas concentration isapproximately 15%, the ammonia, NH₃, gas concentration is approximately20%. The substrate 21 was cooled from the growth temperature,approximately 1050° C., to 600° C. in 5 min.

The nitride semiconductor layers, the p-type second clad layer 26 andthe p-type contact layer 27, among others, thus provided have beenformed by the same growth process and the cooling method as inembodiment 1. Therefore, superior p-type semiconductor layers of lowresistivity have been provided, without undergoing the post annealingtreatment needed for activating the Mg doped in the second clad layer 26and the p-type contact layer.

Next, by using a CVD process, for example, a silicon oxide layer isformed on the p-type contact layer 27 by deposition, a certain specificpattern is provided by a photolithography to form an etching mask, andthen the epitaxial layer is etched through a reactive ion etchingprocess using the mask until the n-type contact layer 23 is exposed.

An n-side electrode 29 is formed selectively on the exposed surface ofn-type contact layer 23 using, for example, an evaporation process; inthe same way, a p-side electrode 28 is formed selectively on the p-typecontact layer 27.

The bottom surface, or the surface opposite to the epitaxial layer, ofsubstrate 21 is ground to make thickness of the substrate 21approximately 100 μm, and the substrate is separated into chips byscribing. Each of the separated chips is fixed on a stem havingelectrodes thereon with the device-bearing surface up. The p-sideelectrode 28 and the n-side electrode 29 of the chip are connected withrespective electrodes of the stem, and then the entire chip is moldedwith a resin to become a finished light emitting diode.

It has been confirmed that the light emitting diode provided through theabove described procedure emits blue light of 470 nm peak wavelengthwhen driven by a 20 mA forward current. The light output was 2.0 mW,forward operation voltage was 4.0 V.

First Modification of Embodiment 2

A light emitting diode having the structure of FIG. 3 was provided bymodifying the cooling time of embodiment 2 to 25 min., where temperatureof the substrate 21 bearing epitaxial layer is lowered from the growthtemperature, approximately 1050° C. to 600° C. The above light emittingdiode emitted blue light of 470 nm peak wavelength when driven by a 20mA forward current. The light output was 0.5 mW, forward operationvoltage was 5.0V.

COMPARATIVE EXAMPLE

A light emitting diode having the structure of FIG. 3 was provided bymodifying the cooling time of embodiment 2 to 40 min., where temperatureof the substrate 21 bearing epitaxial layer is lowered from the growthtemperature, approximately 1050° C., to 600° C. The above light emittingdiode was driven by a 20 mA forward current, but the electric currentdid not flow because of high resistance, hence no light was emitted.

FIG. 4 is a graph showing the hydrogen distribution in the direction ofdepth from the surface with nitride semiconductor light emitting devicesin embodiment 2, in the first modification of embodiment 2 and thecomparative example, measured through SIMS (secondary ion massspectroscopy). The hydrogen concentration is exhibited with respect tothe above-described three different time spans provided for the cooling.In FIG. 4, the regions corresponding to semiconductor layers of FIG. 3are identified by providing the same marks, respectively. Curve 1Arepresents the case of embodiment 2, where the cooling time from thegrowth temperature of approximately 1050° C. to 600° C. is 5 min., curve1B represents the case of the modification where it is 25 min. and curve1C represents the case of the comparative example where it is 40 min.

Curve 1A in FIG. 4, representing the cooling time 5 min., indicates thatthe hydrogen concentration is approximately 3.0×10¹⁹ cm⁻³ at a pointclose to the surface, which concentration decreases along withproceeding towards the substrate (the direction towards the n-typecontact layer 23), and in the second clad layer 26 the hydrogenconcentration becomes almost constant at approximately 1.0×10¹⁹ cm⁻³. Inthe first clad layer 25, the light emitting layer 24 and the n-typecontact layer 23, it is lower than 2.0×10¹⁸ cm⁻³, which is the lowerlimit of detection.

Curve 1B of the first modification, representing the cooling time 25min., indicates that the hydrogen concentration is approximately1.0×10²⁰ cm⁻³ at a point close to the surface, which concentrationdecreases along with proceeding towards the substrate, and in the secondclad layer 26 the hydrogen concentration is almost constant atapproximately 1×10¹⁸ cm⁻³.

Curve 1C of the comparative example, representing the cooling time 40min., indicates that the hydrogen concentration is approximately3.0×10²⁰ cm⁻³ at a point close to the surface, which concentrationdecreases along with proceeding towards the substrate, and in the secondclad layer 26 the hydrogen concentration is almost constant atapproximately 1×10¹⁹ cm⁻³.

FIG. 4 also indicates that in the case where the cooling time is shorterthan 25 min., the hydrogen concentration at the upper surface of p-typecontact layer 27 is within 10 times that in the second clad layer 26.

As the above embodiment 2 and its first modification indicate, asuperior p-type semiconductor layer provided with a low resistivityimmediately after the growth is made available by having the carrier gasused during growth of the p-type second clad layer 26 and the p-typecontact layer 27 to contain hydrogen gas for approximately 5% to 70% incapacity percent, preferably approximately 15% in capacity percent.

Furthermore, the decreasing rate in the hole carrier concentration ofthe p-type semiconductor layer can be suppressed to approximately0%-95%, by setting the cooling time in the cooling process, where it iscooled from the growth temperature higher than 600° C. to approximately600° C., to be shorter than approximately 30 min, preferablyapproximately 5 min., and by forming the atmosphere gas with hydrogengas of approximately 50% or less in capacity percent and ammonia, NH₃,gas of approximately 0.5% or more in capacity percent. As the result, anitride semiconductor light emitting device with low forward operationvoltage and high output is implemented.

Embodiment 3

FIG. 1 is a cross sectional view showing the structure of a p-typenitride semiconductor in accordance with a third exemplary embodiment ofthe present invention. On a substrate 11 made of sapphire, a bufferlayer 12 formed of GaN and a p-type nitride semiconductor layer 13formed of GaN are stacked in the order.

In practice, the substrate 11 is placed in a reaction chamber (notshown) and held by a substrate holder, and then temperature of thesubstrate 11 is raised to approximately 1000° C., and the surface ofsubstrate 11 is cleaned by introducing flow of nitrogen gas and hydrogengas.

The substrate temperature is lowered to approximately 550° C., and thennitrogen gas is introduced as carrier gas, and ammonia, NH₃, andtrimethylgallium (TMG) are supplied to form a buffer layer 12 on thesurface of substrate 11.

The TMG supply to reaction chamber is once suspended and the substratetemperature is raised to approximately 1050° C., and a p-type nitridesemiconductor layer 13 of GaN doped with Mg, which being p-type dopant,is grown on the buffer layer 12, by supplying ammonia, NH₃, TMG andbiscyclopentadienymagnesium (Cp₂Mg), while introducing nitrogen gas andhydrogen gas as the carrier gas.

The TMG and Cp₂Mg supplies to the reaction chamber are suspended, andthen the substrate 11 is cooled from 1050° C. to 700° C., maintainingthe ambient flow of nitrogen gas, hydrogen gas and ammonia, NH₃, gas.Thereafter, the supply of hydrogen gas and ammonia, NH₃, is suspended,and the substrate 11 is cooled down below 100° C., maintaining the flowof nitrogen gas as the ambient gas.

The method for manufacturing p-type nitride semiconductor in embodiment3 features in that the p-type nitride semiconductor layer is cooled inthe cooling process during approximately 950° C.—approximately 700° C.in substrate temperature under a combination of the hydrogenconcentration in the atmosphere and the cooling time, with which theinactivation is hard to occur with the p-type dopant. Furtherdescription on this respect follows.

Regarding the cooling process to be applied to a p-type nitridesemiconductor after it is formed at a substrate temperatureapproximately 950° C. or higher, inventors of the present inventionfound out a fact that the hole carrier concentration in the p-typenitride semiconductor is decreased specifically during a temperaturerange approximately 950° C.—approximately 700° C. in terms of substratetemperature, by hydrogen existing in the ambient.

FIG. 5 is a graph showing the dependence of the hole carrierconcentration in embodiment 3 on the substrate holding temperatureduring the cooling process. Namely, the respective temperatures shown inthe graph represent a temperature at which a substrate, which bears Mgdoped GaN layer formed at a substrate temperature 1050° C., is heldduring the cooling process for 10 min. before it is cooled to roomtemperature; the horizontal axis represents the substrate temperature atwhich it is held for 10 min., while the hole carrier concentration isplotted on the vertical axis. The atmosphere for the cooling process wasprepared with a 20% concentration of ammonia, NH₃, in nitrogen base, intwo versions of different hydrogen concentrations, 30% and 0%. As shownin FIG. 5, in the case of 30% hydrogen concentration, the hole carrierconcentration significantly decreased in the temperatures between 950°C.-700° C., with the lowest at approximately 800° C. The case of 0%hydrogen concentration also exhibited similar trends in the sametemperatures, but amount of the decrease was smaller. Namely, it hasbeen clarified that the hole carrier concentration in p-type nitridesemiconductor decreases in line with concentration of the hydrogenexisting in the atmosphere in the temperature range 950° C.-700° C. Thelowering of the hole carrier concentration is supposed to have beencaused by a p-type dopant contained in the p-type nitride semiconductor,which p-type dopant being inactivated as a result of coupling withhydrogen. The slight lowering occurred also even in the case of 0%hydrogen concentration seems to have been caused by a hydrogen generatedfrom decomposed ammonia, NH₃.

Furthermore, relationship between the hole carrier concentration and thecooling time taken for lowering the substrate temperature fromapproximately 950° C. to approximately 700° C. in the cooling processwas also investigated.

FIG. 6 is a graph showing the dependence of the hole carrierconcentration in embodiment 3 on the substrate cooling time taken forlowering from approximately 950° C. to approximately 700° C. in thecooling process. Namely, Mg doped GaN layers formed at substratetemperature 1050° C. were cooled to 700° C. at various cooling rates,and then down to room temperature; the horizontal axis represents thecooling time taken for lowering the substrate temperature fromapproximately 950° C. to approximately 700° C., while the hole carrierconcentration is plotted on the vertical axis. The cooling rate has beencontrolled to be substantially constant at least in the course of thesubstrate temperature range from approximately 950° C. to approximately700° C. The atmosphere for the cooling process was prepared with a 20%concentration of ammonia, NH₃, in nitrogen base, in four versions ofdifferent hydrogen concentrations, 50%, 30%, 10% and 0%. As seen in FIG.6, the hole carrier concentration decreased along with the lengtheningcooling time with all of the cases of different hydrogen concentrations.Cooling time needed for the hole carrier concentration to becomeapproximately 1×10¹⁶ cm⁻³ at room temperature was calculated from FIG.6; results were, 1.0 min., 1.8 min., 4.1 min. and 15 min. for thehydrogen concentrations 50%, 30%, 10% and 0%, respectively. Namely, inorder to assure the value approximately 1×10¹⁶ cm⁻³ or higher, which isthe requirement to be a practically usable p-type semiconductor, it hasbeen found out that the cooling time for lowering the substratetemperature from approximately 950° C. to approximately 700° C. needs tobe within 1.0 min., 1.8 min., 4.1 min and 15 min., respectively, for thecases of hydrogen concentrations 50%, 30%, 10% and 0%.

FIG. 7 is a graph showing the relationship between the hydrogenconcentration in the atmosphere and the substrate cooling time taken forlowering from approximately 950° C. to approximately 700° C. in thecooling process in a method for manufacturing p-type nitridesemiconductor in accordance with an embodiment of the present invention.

Based on the above results, it can be stated that the combinations ofthe hydrogen concentration in the atmosphere and the cooling time, underwhich combinations the inactivation of p-type dopant is difficult tooccur during the substrate cooling process from approximately 950° C. toapproximately 700° C. that comes after a p-type nitride semiconductor isformed, should fall within a range shown in FIG. 7; in a coordinate (X,Y), X axis representing the hydrogen concentration (%) in atmosphere,while Y axis representing the cooling time (min.) taken for cooling asubstrate from approximately 950° C. to approximately 700° C., the rangeis shown by a region surrounded by points A-B-C-D-E-F, where the pointA(50, 1.0), point B(30, 1.8), point C(10, 4.1), point D(0, 5), pointE(0, 0.5) and point F(50, 0.5). The reason why the lowest limit ofcooling time is specified by a straight line E-F to be 0.5 min.irrelevant to the hydrogen concentration, is that if it is cooled in acooling time shorter than that the p-type nitride semiconductor isliable to be damaged by cracks caused by thermal shock.

Embodiment 4

Likewise in embodiment 3, a p-type nitride semiconductor having thestructure of FIG. 1 is manufactured through a method in accordance witha fourth exemplary embodiment of the present invention.

In the first place, a buffer layer 12 and a p-type nitride semiconductorlayer 13 are formed in the order on a substrate 11 as shown in FIG. 1,in the same way as in embodiment 3.

In the method for manufacturing p-type nitride semiconductor inembodiment 4, the p-type nitride semiconductor layer is cooled when thesubstrate temperature is in the vicinity of approximately 800° C. undera combination of the hydrogen concentration in the atmosphere and thecooling rate, with which the inactivation is hard to occur with thep-type dopant.

As seen in FIG. 5, the inactivation of p-type dopant develops mostsignificantly when the substrate temperature is approximately 800° C. ina range between approximately 950° C. and approximately 700° C.Therefore, in order to maintain the low resistivity property of a p-typenitride semiconductor, it is effective to make the travelling time ofthe substrate at the vicinity of 800° C. shortest possible. In otherwords, the cooling rate at approximately 800° C. should be faster thanthe specified.

From FIG. 6, it is understood that in order to secure the valueapproximately 1×10¹⁶ cm⁻³ or more, which being a requirement to be apractical p-type semiconductor, the cooling rate at the vicinity ofsubstrate temperature 800° C. needs to be 250° C./min., 140° C./min.,61° C./min. and 17° C./min. or faster, respectively, for the cases ofhydrogen concentration 50%, 30%, 10% and 0%.

FIG. 8 is a graph showing the relationship between the hydrogenconcentration in the atmosphere and the substrate cooling rate at thevicinity of 800° C. during the cooling process, in a method formanufacturing p-type nitride semiconductor in accordance with a fourthexemplary embodiment of the present invention.

Based on the above results, it can be stated that the combinations ofthe hydrogen concentration in the atmosphere and the cooling rate, underwhich combinations the inactivation of p-type dopant is difficult tooccur at the substrate temperature approximately 800° C. during thesubstrate cooling process that comes after a p-type nitridesemiconductor is formed, should fall within a range shown in FIG. 8; ina coordinate (X, Y), X axis representing the hydrogen concentration (%)in atmosphere, while Y axis representing the representing the coolingrate (° C./min.) at the vicinity of substrate temperature 800° C., therange is shown by a region surrounded by points O-P-Q-R-S-T, where thepoint O(50, 250), point P(30, 140), point Q(10, 61), point R(0, 17),point S(0, 500) and point T(50, 500). The reason why the upper limit ofcooling rate is specified by a straight line S-T to be 500° C./min.,irrelevant to the hydrogen concentration, is that if it is cooled at acooling rate faster than that a p-type nitride semiconductor is liableto be damaged by cracks due to thermal shock.

The substrate 11 may be made of, besides sapphire, different materialsother than nitride semiconductor, such as SiC, spinel, Si, GaAs, etc.Also, it can be made with GaN or other nitride semiconductor materials.In a case where the substrate 11 is made of such a different material, abuffer layer 12 of GaN, etc. is formed at a low substrate temperatureapproximately 400° C.—approximately 600° C., between the substrate 11and p-type nitride semiconductor layer 13 to be formed thereon, in orderto ease the lattice mismatch between the nitride semiconductor and thesubstrate 11. In a case where the substrate 11 is made of nitridesemiconductor, a p-type nitride semiconductor layer 13 may be formeddirect on the substrate 11, without providing a buffer layer 12.

The p-type nitride semiconductor layer 13 may be provided on a substrate11 on which an n-type nitride semiconductor layer and an active layer ofnitride semiconductor are formed in advance. Thus a device layerstructure having PN junction can be formed.

The p-type nitride semiconductor layer 13 can be formed by introducingthe sources of p-type dopant, nitrogen and Group III sources, whilekeeping the temperature of substrate 11 at approximately 950° C. orhigher. Preferred temperature range for the substrate is approximately950° C.—approximately 1200° C. If the substrate temperature is lowerthan 950° C., the p-type dopant readily couples with hydrogen duringformation of the p-type nitride semiconductor layer 13, making itinactive. This makes formation of a low resistivity p-type nitridesemiconductor layer difficult. If the substrate temperature is higherthan 1200° C., it becomes difficult to form a p-type nitridesemiconductor layer of superior crystal property.

The p-type nitride semiconductor layer 13 may be provided in the form ofa single layer of GaN, AlGaN, InGaN, InAlGaN, etc, or by stacking someof these layers. What is preferred of these is Al_(x)Ga_(1-x)N (0≦x<1),which provides a superior crystal property at a substrate temperatureapproximately 950° C. or higher, or that doped with a very small amountof In.

For the p-type dopant to the p-type nitride semiconductor layer 13, Mg,Zn, Cd, C, etc. may be used. What is preferred of these is Mg, withwhich the p-type conduction is made available with a relative ease.Concentration of the p-type dopant should preferably be not lower than1×10¹⁹ cm⁻³ not higher than 5×10²⁰ cm⁻³. If the concentration of p-typedopant is lower than 1×10¹⁹ cm⁻³, the hole carrier concentration ofp-type nitride semiconductor layer 13 becomes low, and ohmic contactresistance becomes high when forming an electrode on the p-type nitridesemiconductor layer 13. If it is higher than 5×10²⁰ cm⁻³, the crystalproperty of p-type nitride semiconductor layer 13 deteriorates due tothe p-type dopant doped for a high concentration, rendering it difficultto obtain the p-type conduction.

As to the atmosphere in a reaction chamber for forming the p-typenitride semiconductor layer 13, it is preferred that it containshydrogen of about 5%-70% concentration, more preferably 10%-30%concentration. If the concentration is lower than 5%, the crystalproperty of p-type nitride semiconductor layer 13 deteriorates due tolowered atomic migration at the formation surface, and the rate ofp-type dopant taken in the p-type nitride semiconductor layer 13decreases. If the concentration goes beyond 70%, the p-type dopant isinactivated by hydrogen during formation of the p-type nitridesemiconductor layer 13.

In the cooling process, the ammonia, NH₃, concentration in theatmosphere should preferably be maintained to be 5% or higher, at leastby the time when the substrate temperature becomes lower thanapproximately 950° C. If the concentration is lower than 5%, nitrogenreadily separates from the surface of the p-type nitride semiconductor,which readily leads to a deteriorated crystal property at the surface.

Also, in the cooling process, the ammonia, NH₃, concentration in theatmosphere should preferably be 30% or lower during the time when thesubstrate temperature is approximately 950° C.—approximately 700° C. Ifit is higher than 30%, the hole carrier concentration in the p-typenitride semiconductor easily decrease because of increased hydrogengenerated as a result of thermal decomposition of ammonia, NH₃.

CONCRETE EXAMPLE

Now in the following, method for manufacturing p-type nitridesemiconductor of the present invention is described on concreteexamples, referring to the drawings.

Concrete Example 1

A p-type nitride semiconductor was manufactured, the cross sectionalstructure of which is as shown in FIG. 1.

In the first place, a substrate 11 having a mirror-finished main surfacewas placed in a reaction chamber (not shown) and held by a substrateholder, and then temperature of the substrate 11 is raised toapproximately 1000° C., and nitrogen gas was introduced at 5 L/min.,hydrogen gas at 5 L/min. while the substrate 11 was heated forapproximately 10 min. Stains of organic substance and humidity stickingon the surface of substrate 11 were thus removed.

The substrate temperature was lowered to approximately 550° C., and thennitrogen gas was introduced as carrier gas at a flow rate ofapproximately 16 L/min., and ammonia, NH₃, at a flow rate ofapproximately 4 L/min., TMG at a flow rate of approximately 40 μmol/min.to form a buffer layer 12 of GaN for a thickness of approximately 0.03μm on the surface of substrate 11.

The TMG supply to reaction chamber was once suspended and the substratetemperature was raised to approximately 1050° C., and a 2 μm thickp-type nitride semiconductor layer 13 of GaN doped with Mg, p-typedopant, was grown on the buffer layer 12, by introducing nitrogen gas ata flow rate of approximately 12 L/min., hydrogen gas at a flow rate ofapproximately 4 L/min. as the carrier gas, and ammonia, NH₃, at a flowrate of approximately 4 L/min., TMG at a flow rate of approximately 80μmol/min. and Cp₂Mg at a flow rate of approximately 0.2 μmol/min. Mgconcentration in the p-type nitride semiconductor was approximately2×10¹⁹ cm⁻³. The flow rate of hydrogen gas includes the hydrogen gasneeded for vaporizing the TMG and the Cp₂Mg.

The supply of TMG and Cp₂Mg to the reaction chamber was suspended, andthen the substrate 11 was cooled from 1050° C. to 950° C. inapproximately 0.5 min., while introducing nitrogen gas at a flow rate ofapproximately 12 L/min., hydrogen gas at a flow rate of approximately 4L/min. and ammonia, NH₃, at a flow rate of approximately 4 L/min. as theambient gas.

And then, hydrogen gas was suspended, and nitrogen gas was supplied at aflow rate of approximately 16 L/min., ammonia, NH₃, at a flow rate ofapproximately 4 L/min. as the ambient gas, while temperature ofsubstrate 11 was lowered from 950° C. to 700° C. in approximately 1.2min. The rate of cooling the substrate 11 at the vicinity of 800° C. wasapproximately 210° C./min. After that, the ammonia, NH₃, supply wassuspended, and nitrogen gas was kept on flowing as the ambient gas at aflow rate of approximately 20 L/min. until the substrate temperaturebecomes lower than 100° C.

After the cooling was finished, the substrate 11 provided with p-typenitride semiconductor layer 13 was taken out of the reaction chamber.The substrate 11 was separated into individual chips of 5 mm squarewithout undergoing post annealing. The chip was measured in the halleffect by the Van der Pauw method; the result showed that the holecarrier concentration was 1.6×10¹⁷ cm⁻³, and there was a superior p-typesemiconductor layer of low resistivity. The hole carrier concentrationof the p-type nitride semiconductor layer 13 before cooling wasestimated to be approximately 2.0×10¹⁷ cm⁻³ as the result ofextrapolating the zero cooling time in FIG. 6. Accordingly, the decreaseof the hole carrier concentration was suppressed within approximately20% during the cooling process.

Concrete Example 2

A p-type nitride semiconductor of the present example 2 was manufacturedin the same procedure as in example 1, except that the conditions ofatmosphere during cooling process were modified.

Practically described, after the p-type nitride semiconductor layer 13was formed, the supply of TMG and Cp₂Mg to the reaction chamber wassuspended, and then the substrate 11 was cooled from 1050° C. to 700° C.in approximately 1.7 min., while supplying nitrogen gas at a flow rateof approximately 12 L/min., hydrogen gas at a flow rate of approximately4 L/min. and ammonia, NH₃, at a flow rate of approximately 4 L/min. asthe ambient gas. It took approximately 0.5 min. for the temperature ofsubstrate 11 to come down from 1050° C. to 950° C., approximately 1.2min. from 950° C. to 700° C. The rate of cooling the substrate 11 at thevicinity of 800° C. was approximately 210° C./min.

After the temperature of substrate 11 became lower than 700° C., supplyof the hydrogen gas and ammonia, NH₃, was suspended, and nitrogen gaswas kept on flowing as the ambient gas at a flow rate of approximately20 L/min. until the substrate temperature becomes lower than 100° C.

After the cooling was finished, the substrate 11 provided with p-typenitride semiconductor layer 13 was taken out of the reaction chamber.The substrate 11 was separated into individual chips of 5 mm square. Thechip was measured in the hall effect. Result of the measurement showedthat the hole carrier concentration was approximately 4.6×10¹⁶ cm⁻³. Thehole carrier concentration of the p-type nitride semiconductor layer 13before cooling is assumed to be approximately 2.0×10¹⁷ cm⁻³.Accordingly, the decrease of positive hole carrier concentration duringthe cooling process was suppressed within approximately 77%.

Comparative Example 1

A p-type nitride semiconductor of the present comparative example 1 wasmanufactured in the same procedure as in the concrete example 2, exceptthat the cooling time (or cooling rate) during cooling process weremodified.

Practically described, after the p-type nitride semiconductor layer 13was formed, the supply of TMG and Cp₂Mg to the reaction chamber wassuspended, and then the substrate 11 was cooled from 1050° C. to 700° C.in approximately 5.6 min., while supplying nitrogen gas at a flow rateof approximately 12 L/min., hydrogen gas at a flow rate of approximately4 L/min. and ammonia, NH₃, at a flow rate of approximately 4 L/min. asambient gas. It took approximately 1.6 min. for the temperature ofsubstrate 11 to come down from 1050° C. to 950° C., approximately 4.0min. from 950° C. to 700° C. The rate of cooling the substrate 11 at thevicinity of 800° C. was approximately 63° C./min.

After the temperature of substrate 11 became lower than 700° C., supplyof the hydrogen gas and ammonia, NH₃, was suspended, and nitrogen gaswas kept on flowing as the ambient gas at a flow rate of approximately20 L/min. until the substrate temperature becomes lower than 100° C.

After the cooling was finished, the substrate 11 provided with p-typenitride semiconductor layer 13 was taken out of the reaction chamber.The substrate 11 was separated into individual chips of 5 mm square. Thechip was measured in the hall effect. Result of the measurement showedthat the hole carrier concentration was approximately 2×10¹⁵ cm¹³, ahigh resistivity.

The hole carrier concentration of the p-type nitride semiconductor layer13 before cooling is assumed to be approximately 2.0×10¹⁷ cm⁻³.Accordingly, the hole carrier concentration decreased by approximately99% during the cooling process.

Concrete Example 3

FIG. 9 is a cross sectional view showing the structure of p-type nitridesemiconductor in accordance with other exemplary embodiment of thepresent invention.

In the present concrete example, a nitride semiconductor light emittingdevice of FIG. 9 was manufactured, in which a p-type nitridesemiconductor layer was disposed at the uppermost.

The nitride semiconductor light emitting device comprises a first n-typeclad layer 22 of non-doped GaN, a second n-type clad layer 23 ofnon-doped AlGaN, a light-emitting layer 24 of non-doped InGaN, anintermediary layer 25 of non-doped GaN and a p-type clad layer 26 ofMg-doped AlGaN stacked in the order on a substrate 21 of Si-doped n-typeGaN. On the p-type clad layer 26 is a p side electrode 27 formed of Ptand Au stacked in the order, while an n side electrode 28 is formed ofTi and Au which is disposed on the exposed region of the substrate 21.Thus the present light emitting device is a light emitting diode forminga PN junction with the light-emitting layer 24 in between.

Method for manufacturing the light emitting diode of the aboveconfiguration is described in the following.

In the first place, a substrate 21, which is made of GaN, doped with Sito provide an n-type property and having a mirror-finished surface, wasplaced in a reaction chamber (not shown) and held by a substrate holder.Temperature of the substrate 21 was raised to approximately 1100° C.,and the substrate 21 was heated for approximately 1 min., whilesupplying nitrogen gas at a flow rate of 4 L/min., hydrogen gas at aflow rate of 4 L/min. and ammonia, NH₃, at a flow rate of 2 L/min. onthe substrate 21. Stains of organic substance and humidity sticking onthe surface were thus removed.

The substrate temperature was maintained at approximately 1100° C., andnitrogen gas was introduced at a flow rate of approximately 13 L/min.,hydrogen gas at a flow rate of approximately 3 L/min. as carrier gas,and ammonia, NH₃, at a flow rate of approximately 4 L/min., TMG at aflow rate of approximately 80 μmol/min. to form the first n-type cladlayer 22 of non-doped GaN for a thickness of 0.5 μm.

After the first n-type clad layer 22 was formed, temperature of thesubstrate 21 was maintained at approximately 1050° C., and nitrogen gaswas introduced at a flow rate of approximately 15 L/min., hydrogen gasat a flow rate of approximately 3 L/min. as carrier gas, and ammonia,NH₃, at a flow rate of approximately 2 L/min., TMG at a flow rate ofapproximately 40 μmol/min. and trimethylaluminum (TMA) at a flow rate ofapproximately 3 μmol/min. to form a 0.05 μm thick second n-type cladlayer 23 of non-doped Al_(0.05)Ga_(0.95)N.

After the second n-type clad layer 23 was formed, supply of the TMG andTMA was suspended, temperature of substrate 21 was lowered toapproximately 700° C., and maintained at this level. As the carrier gas,nitrogen gas was supplied at a flow rate of approximately 14 L/min.,ammonia, NH₃, at a flow rate of approximately 6 L/min., TMG at a flowrate of approximately 4 μmol/min. and trimethylindium (TMI) at a flowrate of approximately 1 μmol/min. to form an SQW light-emitting layer 24of non-doped In_(0.15)Ga_(0.85)N for a thickness of 0.002 μm.

After the light-emitting layer 24 was grown, supply of the TMI wassuspended, and nitrogen gas was kept on flowing as carrier gas at a flowrate of approximately 14 L/min., and ammonia, NH₃, at a flow rate of 6L/min., TMG at a flow rate of 2 μmol/min. on the substrate 21 raisingthe substrate temperature towards 1050° C. In this way, the intermediarylayer 25 of non-doped GaN was formed for a thickness of 0.004 μm.

After the substrate 21 temperature reached at 1050° C., nitrogen gas wassupplied at a flow rate of approximately 14 L/min., hydrogen gas at aflow rate of approximately 4 L/min. as carrier gas, and ammonia, NH₃, ata flow rate of 2 L/min., TMG at a flow rate of 40 μmol/min., TMA at aflow rate of 3 μmol/min. and Cp₂ Mg at a flow rate of 0.4 μmol/min. togrow a p-type clad layer 26 of Mg-doped Al_(0.05)Ga_(0.95)N for athickness of 0.2 μm. Mg concentration in the p-type clad layer 26 wasapproximately 8×10¹⁹ cm⁻³.

After the p-type clad layer 26 was grown, supply of the TMG, TMA and CP₂Mg was suspended, and temperature of the substrate 21 was lowered from1050° C. to 950° C. in approximately 0.5 min. while introducing nitrogengas at a flow rate of approximately 14 L/min., hydrogen gas at a flowrate of approximately 4 L/min. and ammonia, NH₃, at a flow rate ofapproximately 2 L/min. as the ambient gas.

Supply of the hydrogen gas was suspended, and then temperature of thesubstrate 21 was lowered from 950° C. to 700° C. in approximately 1.2min., while supplying nitrogen gas at a flow rate of approximately 18L/min., ammonia, NH₃, at a flow rate of approximately 2 L/min. as theambient gas. The rate of cooling the substrate 21 at the vicinity of800° C. was approximately 210° C./min.

After the temperature of substrate 21 became lower than 700° C., supplyof the ammonia, NH₃, was suspended, while nitrogen gas was kept onflowing as the ambient gas at a flow rate of approximately 20 L/min.until the substrate 21 temperature became lower than 100° C. Then thesubstrate 21 was taken out of the reaction chamber.

The nitride semiconductor layers thus provided, the p-type clad layer26, among others, proved themselves to be superior p-type semiconductorlayers of low resistivity, without undergoing the post annealing processfor activating the doped Mg.

Next, over the surface of nitride semiconductor having a stacked-layerstructure thus formed, a SiO₂ layer was deposited through a CVD process,without applying any post annealing. A rectangular patterning wasprovided thereon through photolithography and wet etching process, and aSiO₂ etching mask was formed. By using a reactive ion etching process,the p-type clad layer 26, the intermediary layer 25, the light-emittinglayer 24, the second n-type clad layer 23, the first n-type clad layer22 and a part of the substrate 21 were removed in a direction reverse tothe stacking direction of layers, until the substrate 21 was etched offfor a depth of approximately hum. Thus the surface of substrate 21 wasexposed in part. On a part of the exposed surface of substrate 21, an nside electrode 28 was formed by stacking a 0.1 μm thick Ti and a 0.5 μmthick Au, through photolithography and evaporation process. Afterremoving the SiO₂ etching mask by wet etching process, a p sideelectrode 27 formed of a 0.3 μm thick Pt and a 0.5 μm thick Au wasprovided covering most part of the surface of the p-type clad layer 26by using photolithography and evaporation process. The substrate 21 wasadjusted in the thickness to a 100 μm thick by back grinding, and thenseparated into individual chips by scribing.

A nitride semiconductor light-emitting device was thus provided in thestructure as illustrated in FIG. 9.

The light-emitting device was mounted, with the chip having theelectrodes down, on a Si diode on which a couple of positive andnegative electrodes are provided. They are connected to each other withan Au bump. The light-emitting device was mounted so that the p sideelectrode 27 and the n side electrode 28, respectively, are connectedwith the negative electrode and the positive electrode of the Si diode.The Si diode bearing the light-emitting device was mounted and fixed ona stem using an Ag paste, the positive electrode of Si diode wasconnected to an electrode on the stem with a wire, and then molded withresin to complete a finished light emitting diode. The light emittingdiode was driven by a 20 mA forward current, and it emitted blue lightof 470 nm peak wavelength exhibiting an even light emission from thereverse surface of substrate 21. The light output was 4 mW, forwardoperation voltage was 3.4 V.

As described in the above, in the present concrete example, a p-typesemiconductor layer of low resistivity and superior quality is formed asthe p-type clad layer 26 in the p-type nitride semiconductor layerforming process; and in the cooling process, it can be cooled whilepreserving the low resistivity property of the p-type clad layer 26. Asa result, a nitride semiconductor light-emitting device operating on alow voltage and yielding a high output is made available without needingany post annealing treatment or other such specific processing.

In a method for manufacturing p-type nitride semiconductor of thepresent invention, a low resistivity p-type nitride semiconductor layeris formed on a substrate in an atmosphere containing hydrogen for acertain specific degree at which the inactivation of p-type dopant canbe well suppressed; and the p-type nitride semiconductor layer thusformed is cooled in a certain specific cooling time, or atmosphere, sothat the hole carrier concentration of the p-type nitride semiconductorlayer decreases in a manner where the low resistivity property isreasonably preserved. As a result, a p-type nitride semiconductor ofsuperior crystal property is made available without needing any postannealing treatment.

In a method for manufacturing p-type nitride semiconductor of thepresent invention, a low resistivity p-type nitride semiconductor layeris formed on a substrate, and the p-type nitride semiconductor layer iscooled, during a certain specific substrate temperature range in thecooling process, under certain specific combination of the hydrogenconcentration in the atmosphere and the cooling time, or the hydrogenconcentration in the atmosphere and the cooling rate, under whichspecific combination it is hard for the p-type dopant to becomeinactivated. As a result, a p-type nitride semiconductor of lowresistivity and superior crystal quality is made available withoutneeding any post annealing treatment or other such specific processing.

Furthermore, in the method of present invention, the manufacturingprocess of p-type nitride semiconductor can be simplified, so the costof manufacturing a nitride semiconductor device incorporating a p-typenitride semiconductor can be reduced.

1-10. (canceled)
 11. A p-type nitride semiconductor grown on a substrateat a temperature of 600° C. or higher, wherein the hole carrierconcentration immediately after the cooling equals to approximately5%-100% of the hole carrier concentration at said growth temperature.12. A p-type nitride semiconductor grown on a substrate at a temperatureof 600° C. or higher, the upper surface of said p-type nitridesemiconductor being exposed, wherein the hydrogen concentration at thevicinity of upper surface of said p-type nitride semiconductor equals to1-10 times that in the inside of said p-type nitride semiconductor